1. Fields of the Invention
The present invention relates to a method for mirror polishing the chamfered peripheral part of a wafer and an apparatus for the same.
2. Description of the Related Art
The manufacturing process for mirror wafers of such as silicon semiconductor or compound semiconductor, is outlined as follows. A single crystal ingot first manufactured by pulling method or the like, is sliced into disc shape to obtain as-sliced wafers Subsequently, a lapping process is conducted in order to produce a flatness of the surface and a parallelism between the front and back surfaces of the as-sliced waters by grinding extremely uneven layers produced in the slicing process As the wafer having gone through these mechanical processes have a damage layer, that is, a deteriorated layer by mechanical impact on its surface, that layer is removed by chemical etching. To finish the wafer process, the wafer is polished to a mirror surface.
Thus produced semiconductor mirror wafer is carried into next process of device manufacturing. The device manufacturing process includes a substrate process, a wiring process and a test process. An essential substrate structure of a device is formed on a wafer in the substrate process. The processed substrate is put into a next wiring process for forming a further device structure such as wiring patterns. The substrate is cut into device chips that are delivered to an assembly process.
Most of these works for processing are wafers as seen through the processes from a single crystal ingot to a semiconductor device. Wafers are used as works not only in the mirror wafer manufacturing process but in the device fabricating process also. Thus, wafers are handled even in both the substrate process and the wiring process of device fabrication Operations utilizing the peripheral part of wafers are frequently repeated therethrough. Consequently, if the wafer has a sharp peripheral edge of a right angle as it is cut, it is liable to give rise to mechanical defects such as chips and cracks which further cause particles attaching on the surface of the wafer to result in a bad production yield of the subsequent process.
If the peripheral part of the wafer has a coarse surface, chemicals used in an etching process are not perfectly cleaned so as to affect the subsequent process defectively. Further, when the single crystal wafer is subjected to epitaxial growth on its major surface in order to increase a value added as a product in a wafer manufacturing industry, the irregular and defective crystal arrangement of the peripheral surface may generate epitaxial crown (continuous protuberant crystal growth) on the edge part of the major surface or plurality of protrusions called nodule. Therefore, great emphasis is laid not only upon processing of the main surface but also processing of the peripheral part. It is common that the peripheral part of the wafer these days is polished like a mirror surface in addition to being chamfered as a highly finished state of the surface to avoid chipping.
Thus, in a chamfering process for the peripheral part of the wafer, the peripheral part is first ground in accordance with the sectional profile of it before a polishing process in order to prepare a chamfered form thereof. Fragmental abrasive grains broken from the grindstone or the deformation of the grindstone in this process causes deep scratches on the surface. Therefore, the surface needs to be polished at least as deep as to remove the scratches.
The peripheral parts of wafers are often polished with a tape having abrasive grains thereon in order to lighten the load on the polishing process using polishing cloth As the process with the tape brings more damages on the surface than the process with the polishing cloth, polishing with polishing cloth need to be done carefully after all to get rid of the damages perfectly.
The chamfered peripheral part of the wafer consists of different planes of crystal orientation though the major surface has a definite single plane. As a reactivity of alkaline etching solution to silicon differs according to the crystal orientation of silicon, that is anisotropic in nature, surface roughness is different by the positions of the chamfered part of the water when the chamfered part is etched by alkaline etching. As shown in FIG. 1, the chamfered part of the (100) wafer have {100} planes or {110} planes at 45-degree angular intervals. When it is etched, though the chamfered part of {110} plane position becomes severely rough, the chamfered part of {100} plane position dose not become much so. Such a wafer needs to be polished over the chamfered part based on its surface roughness of the {110} plane position.
Up to now, the chamfered peripheral part of a wafer has usually been polished by a process of a single condition that is commonly called one-step polishing. On the contrary, the major surface of a wafer has ordinarily polished by a plurality of processes of different conditions, that is multi-step polishing processes, in order to strictly pursue both the removal of damages induced by processing and the improvement for the surface roughness. That is because the improvement can not be attained by a single condition as the major surface has different characteristics of unevenness such as flatness, waviness, ripple or haze, the major surface has been polished with the different rate of polishing at each step so as to meet each problem. On the other hand, the problems are roughness and damages induced by processing as far as the peripheral part of the wafer is concerned. Thus the peripheral part of a wafer have been hitherto polished by one step.
Such an apparatus as shown in FIG. 4 have been used for polishing the chamfered peripheral part of a wafer. The apparatus has a rotating drum affixed with a polishing cloth thereon, a wafer rotating part which press the chamfered peripheral part of the wafer against the rotating drum while rotates the wafer thereon and a nozzle which supplies polishing slurry to the contact spot of the polishing cloth to the chamfered peripheral part of the wafer.
As requirement for high-quality of the peripheral part of a wafer becomes strict, the high level of improvement on the roughness and damages induced by processing is also required. Therefore polishing time of the peripheral part becomes so long that a problem arises regarding a life of polishing cloth. Consequently, these issues give rise to the necessity of the improvement in terms of productivity and cost in the process. In a polishing process with a single condition, the peripheral part has often been polished excessively in order to equalize different surface spots of different surface roughness, which has led to reduction of productivity.